Individual human development

Individual human developmentThe individual development of the organism (ontogenesis) covers the period from the moment of fertilization and the appearance of the organism in the form of a zygote and until its death.

Ontogenesis is a state  continuous changes, updates, the emergence of new cells, functions, activities. This is a progressive process, in which inevitably and naturally simple quantitative changes lead to fundamental qualitative changes. Changing the position of the body from horizontal to vertical with the start of walking, the first words and further rapid development of speech, the transformation of a child into a teenager during puberty - these are some stages in human development, each of which is prepared by a previous course of growth and development.

Ontogenesis is carried out on the basis of a hereditary program obtained from the parents' cells that entered into fertilization. During the implementation of hereditary information, the body forms and individual morphological, physiological and biochemical properties that make up the phenotype of the individual. In the process of development under the influence of factors of external and internal environment, the body naturally changes its characteristics, remaining, nevertheless, a complete system. In this regard, the phenotype is commonly understood to be the totality of the properties of an organism throughout its entire individual development, at each stage of which there are specific features.

The whole cycle of individual development is subdivided into a number of periods (Table 3), each of which is characterized by its own peculiarities of morpho-functional changes in the body. Individual differences in mental and physical development can vary widely among children and adolescents of the same calendar (passport) age. Therefore, doctors, psychologists and teachers have to constantly take into account not only the calendar age of the child, but the degree of his biological development. The inconsistency of the biological age of the passport is not uncommon.

Ontogenesis consists of two interrelated and interdependent processes - growth and development (differentiation). Growth is a quantitative change associated with an increase in the size of cells, their number, the mass of both individual organs and tissues, and the whole organism. Development is a qualitative change, morphological differentiation, specialization of tissues and organs, their functional improvement.

Table 3

Age periodization of human ontogenesis

The name of the period of ontogenesis


Antenatal (prenatal, intrauterine) stage of development


Embryonic period

0–2 months of antenatal development


Fetal period

2–9 month of antenna development

Postnatal (postpartum) stage of development


Perinatal period

28th week of antenatal development - the first 10 days of postnatal development


Neonatal period

10 days - 1st month


Chest age

From 1st to 12th month


Early childhood (period of "first rounding")

1-3 year


The first childhood (the period of the "first draw")

4–7 years old


Second childhood (the period of the "second rounding")

8–12 years old (boys)

8-11 years old (girls)


Teenage (puberty) period (“second stretching”)

13–16 years old (boys)

12–15 years old (girls)


Adolescence (juvenile period)

17–21 years old (young men)

16–20 years old (girls)


Mature age, I period

22–35 years old (men)

21–35 years old (women)

Mature age, II period

35–60 years old (men)

35–55 years old (women)


Elderly age

61–74 years old (men)

56–74 years (women)


Old age

75–90 years old



90 years and older